Ran is a small, nuclear Ras-like GTPase that is required for nuclear transport and cell cycle control. To investigate the mechanism of Ran's function, we have studied proteins that interact with Ran in Xenopus egg extracts in two ways. First, we have examined the interactions between Ran and a stable complex that contains three proteins: p340RanBP2, p88RanGAP1 and p18Ubc9. p340RanBP2 is a large protein of the nuclear pore complex (NPC). p88RanGAP1 is a modified form of the GTPase activating protein for Ran, RanGAP1, which has been conjugated with a small ubiquitin-related protein, SUMO-1. SUMO-1 conjugation of RanGAP1 is required for its association with p340RanBP2. RanGAP1-SUMO-1 conjugation is a novel mechanism through which nuclear transport may be regulated via regulation of the Ran GTPase pathway. p18Ubc9 is a protein with homology to E2 ubiquitin conjugation enzymes. We have found that p18Ubc9 acts as an E2-like enzyme for SUMO-1 conjugation, but not for ubiquitin conjugation. Our results suggest that the pathway for SUMO-1 conjugation may be mechanistically similar to the ubiquitin conjugation pathway, but that it uses a distinct set of enzymes and regulatory mechanisms. Second, we have established an in vitro system to examine the role of RanBP1, a guanine nucleotide dissociation inhibitor of Ran, in nuclear assembly, nuclear transport and DNA replication. We have used this system to examine the interactions between RanBP1 and RCC1, the guanine nucleotide dissocation inhibitor for Ran. Our analysis of Ran-interacting proteins is complemented by an analysis of the Xenopus homologue of Rae1, a gene required for both RNA export and regulation of the G2/M transition in yeast.